1. Field of the Invention
The present invention relates to high corrosion resistant equipment for a plant having the lining structure using high corrosion resistant metal materials, and more particularly to high corrosion resistant equipment for a plant which can be operated for a long period by suppressing breaking of the lining.
2. Description of the Related Art
With respect to plant equipment which uses a gas or a liquid having high corrosiveness, a reactor, piping and the like which constitute the equipment are required to maintain soundness of materials against corrosion throughout a use period. A material which is exposed to a corrosive environment is selected by evaluating a use period and a corrosion resistant life while also taking economic efficiency into consideration. However, in a harsh corrosion environment, a high corrosion resistant material whose raw material cost and manufacturing cost are high is used. Particularly, when the large structure is manufactured using only a high corrosion resistant material, a raw material cost and a manufacturing cost are increased, and there also arises a difficulty that it is necessary to increase a plate thickness when strength of the material is found to be low in a structural strength designing. There also may be a case where the structure cannot be manufactured with a currently available manufacturing technique. In view of the above-mentioned circumstances, it may be possible to adopt a method where corrosion resistance against a corrosive environment is imparted to the structure by covering a surface of a structural material such as a steel material for maintaining structural strength with coating or lining made of a high corrosion resistant material.
As the method where the surface of the structural material such as the steel material for maintaining structural strength is covered with the high corrosion resistant material for imparting corrosion resistance against the corrosion environment to the structure, there have been conventionally adopted high corrosion resistant material coating which is obtained by performing welding using a high corrosion resistant material welding material or by performing thermal spraying of a high corrosion resistant powdery material, and lining which is obtained by joining a corrosion resistant material plate material to a structural material by rolling or explosion cladding.
Zirconium, a zirconium alloy, titanium, and a titanium alloy (hereinafter referred to as zirconium, titanium and the like) exhibit excellent corrosion resistance and hence, these materials are used in a chemical product manufacturing plant and a chemical processing plant. In joining zirconium, titanium or the like, oxygen or nitrogen which is mixed into a joining portion from atmospheric air at the time of welding forms oxide or nitride thus causing embrittlement of the joining portion. Accordingly, it is necessary to perform welding under a high atmosphere control such as in the inside of a vacuum chamber or in the inside of an inert gas using a shield box for strictly shielding the atmosphere from atmospheric air. Japanese patent 3562014 (patent document 1) discloses the shield box structure or shielding arrangement used at the time of welding.
Friction stir welding is a method where a tool made of a material harder than a material to be joined is inserted into a joining portion of the material to be joined while being rotated, and the material to be joined is joined by friction heat generated between the tool and the material to be joined. This joining is solid-phase joining and hence, it has been known that the deformation of the material to be joined which is caused by joining and a thermal effect imparted to a material to be joined are small compared to welding. As a means for improving quality of a surface, there is a case where friction stirring is also used in addition to joining. JP-A-2006-255711 (patent document 2) discloses, for example, a method where a high corrosion resistant material plate material is joined to a structural material by friction stir welding as a lining. JP-A-2000-301363 (patent document 3) and JP-A-2002-248583 (patent document 4) disclose a method where active metal such as zirconium or titanium is joined to a material to be joined by friction stir welding in a non-oxidizing inert gas atmosphere, thus suppressing the oxidation of a surface of the material to be joined in joining.
In plant equipment which uses a lining made of a high corrosion resistant material, a progress of corrosion is prevented by the lining. Accordingly, when the lining is broken so that a through hole is formed, an outer structural material is exposed to a corrosion environment through the through hole and thereby the equipment may be heavily damaged.
The joining portion between the lining and the structural material differs from a lining base material in material quality under the influence of joining. Further, a tensile residual stress exists in the joining portion and the joining portion is formed into a shape by which a stress is liable to be concentrated structurally in many cases. Accordingly, the joining portion is liable to be broken in various damage modes such as high cycle fatigue caused by vibrations during an operation, low cycle fatigue caused by a start stop operation, stress corrosion cracking or corrosion fatigue due to the superposition of the fatigue with a corrosion environment during an operation. Particularly, when there is a large difference in thermal expansion coefficient between the lining material and the structural material and the difference in temperature in the plant equipment between a period during which plant equipment is stopped and a period during which the plant equipment is operated, a possibility that the joining portion is broken due to a low cycle fatigue caused by a start stop operation becomes high. The joining portion where the lining and the structural material are directly joined to each other forms a dissimilar joint and hence, there is a possibility that a diluted portion is embrittled due to the precipitation of different phases at the time of welding or at the time of operating a plant and thereby the reliability of the joining portion is low in many cases. Depending on a material to be used for forming the lining, the joining portion formed by welding may have a coarse solidification structure. In such a case, the joining portion becomes an undermatching joint where strength of the joining portion is low with respect to a lining base material is formed, thus giving rise to a possibility that the joining portion is largely deformed when a tensile stress is applied to the joining portion so that the joining portion is broken prior to the breaking of the base material.